This invention relates to fused silica and optical members made from fused silica. More particularly, the invention relates to fused silica and optical members made from fused silica containing increased levels of aluminum.
As practiced commercially, fused silica optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates and windows, are typically manufactured from bulk pieces of fused silica made in large production furnaces. Bulk pieces of fused silica manufactured in large production furnaces are known in the art as boules or ingots. Blanks are cut from boules or ingots, and finished optical members are manufactured from glass blanks, utilizing manufacturing steps that may include, but are not limited to, cutting, polishing, and/or coating pieces of glass from a blank. These optical members are used in various apparatus employed in environments where they are exposed to high-power ultraviolet light having a wavelength of about 360 nm or less, for example, an excimer laser beam or some other high-power ultraviolet laser beam. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser fabrication equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
In overview, boules are manufactured by reacting silicon-containing gas molecules in a flame to form silica soot particles. The soot particles are deposited on the hot surface of a rotating or oscillating body where they consolidate to the glassy solid state. In the art, glass making procedures of this type are known as vapor phase hydrolysis/oxidation processes, or simply as flame deposition processes. The term xe2x80x9cboulexe2x80x9d is used herein with the understanding that the term xe2x80x9cboulexe2x80x9d includes any silica-containing body formed by a flame deposition process.
Boules typically having diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm) and larger can be routinely produced in large production furnaces. Multiple blanks are cut from such boules and used to make the various optical members referred to above. The principal optical axis of a lens element made from such a blank will also generally be parallel to the boule""s axis of rotation in the furnace. For ease of reference, this direction will be referred to as the xe2x80x9caxis 1xe2x80x9d or xe2x80x9cuse axisxe2x80x9d. Measurements made in a direction perpendicular to the axis 1 or use axis will be referred to as xe2x80x9coff-axisxe2x80x9d measurements.
As the energy and pulse rate of lasers increase, the optical members which are used in conjunction with such lasers, are exposed to increased levels of laser radiation. Fused silica members have become widely used as the manufacturing material of choice for optical members in such laser-based optical systems due to their excellent optical properties and resistance to laser induced damage.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength excimer lasers operating in the UV and deep UV (DUV) wavelength ranges, which includes lasers operating at about 193 nm and 248 nm wavelengths. Excimer laser systems are popular in microlithography applications, and the shortened wavelengths allow for increased line densities in the manufacturing of integrated circuits and microchips, which enables the manufacture of circuits having decreased feature sizes. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies in the beam due to the fact that each individual photon is of higher energy. In such excimer laser systems, fused silica optics are exposed to high energy photon irradiation levels for prolonged periods of time resulting in the degradation of the optical properties of the optical members.
It is known that laser-induced degradation adversely affects the performance of fused silica optical members by decreasing light transmission levels, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Over the years, many methods have been suggested for improving the optical damage resistance of fused silica glass. It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting process, plasma CVD process, electrical fusing of quartz crystal powder, and other methods, are susceptible to laser damage to various degrees.
One of the known methods for reducing absorption levels and improved transmission in the glass is to reduce total metal impurity levels of metals such as sodium, aluminum, and iron. In the past, fused silica glass has been manufactured and sold by the assignee of the present invention having aluminum impurity as high as 50 parts per billion (ppb), sodium levels as high as 100 ppb, and an internal transmission of no greater than 99.4%/cm at 193 nm. One known way of reducing metals impurities in the glass involves treating the refractory materials used in the fused silica production furnace with a halogen gas. Further details on this method are described in U.S. Pat. No. 6,174,509. Another known method of improving the transmission and durability of fused silica optical members is disclosed in U.S. Pat. No. 6,174,830, which discloses annealing silica glass members for 10 or more hours at 1000xc2x0 C. so that the hydrogen content of the member is 5xc3x971018 molecules/cm3 or less. While the method in U.S. Pat. No. 6,174,830 is advantageous in that it produces optical members having excellent properties, the annealing process takes a considerable amount of time and expense to produce such members after formation of the boule.
Fused silica members can also exhibit transient absorption. As described in the article xe2x80x9cTransient absorption in excimer-exposed silica,xe2x80x9d by Charlene Smith, Nicholas Borrelli and Roger Araujo, Applied Optics, Vol. 39, No. 31, 5778-5784 (Nov. 1, 2000), the contents of which are incorporated herein by reference, transient absorption can take two forms. In one form, the transmittance of glass in the UV region recovers somewhat when the irradiation source is removed and redarkens quickly when reexposed to light. In the second form, absorption occurs upon the initial irradiation of the glass, and this absorption decreases with constant illumination of the optical member. This type of transient absorption will be referred to herein as the xe2x80x9cabsorption spike.xe2x80x9d This absorption spike is problematic in that to avoid the undesirable effects of absorption changes in an optical member, a manufacturer of optical equipment such as a stepper lens machines must expose the optical members to a sufficient number of pulses to xe2x80x9cwork throughxe2x80x9d the absorption spike and reduce to the absorption value. This exposure process requires optical equipment manufacturers to devote time and resources to xe2x80x9cwork throughxe2x80x9d the absorption spike to reduce absorption to an acceptable level.
The presence or the absence of the absorption spike has been traced to the level of molecular hydrogen dissolved in the glass. Typically, optical members containing high amounts of hydrogen, for example, a concentration of 1019 molecules/cm3, do not have a measurable absorption spike. Accordingly, manufacturing of optical members with high amounts of molecular hydrogen is one way of reducing this absorption spike.
It would be desirable to provide a fused silica glass articles that exhibited improved transmission and a decreased absorption spike. It would be advantageous if such fused silica glass articles could be provided without having to resort to expensive and time consuming treatments such as annealing or prolonged irradiation after formation of the glass boules or blanks used to make fused silica optical members.
The invention relates to fused silica glass articles. As used herein, the term xe2x80x9cfused silica glass articlexe2x80x9d includes the boule or the bulk piece of fused silica produced in a furnace, blanks cut from a boule, and fused silica optical members manufactured from blanks of fused silica. The production of fused silica optical members may involve finishing steps including, but not limited to cutting, grinding, polishing and/or coating the piece of fused silica glass.
According to one embodiment of the present invention, fused silica articles are provided having high resistance to optical damage by ultraviolet radiation in the ultraviolet wavelength range, particularly in the range between 190 and 300 nm. In one embodiment of the invention, the fused silica glass articles of the present invention contain at least about 50 ppb of aluminum and have a minimum internal transmission of 99.5%/cm at a wavelength of 193 nm. In one embodiment, the aluminum is doped into the fused silica glass article. According to another embodiment, the amount of aluminum present in the optical member is greater than about 100 ppb, and in another embodiment, the aluminum is present in an amount between about 200 and 400 ppb.
In other embodiments, the minimum internal transmission of the glass articles is greater than or equal to 99.65%/cm at a wavelength of 193 nm. In certain embodiments, glass articles are produced that have a minimum internal transmission greater than or equal to 99.75%/cm at a wavelength of 193 nm.
The fused silica articles of the present invention also exhibit a lowered absorption change when compared with fused silica articles containing aluminum at less than 50 ppb levels. Fused silica articles in accordance with the present invention exhibit an absorption change less than about 0.0006/cm (base 10) when irradiated with a 193 nm laser having a fluence of at least about 0.97 mJ/cm2/pulse. In a preferred embodiment, the fused silica articles of the present invention exhibit an absorption change less than about 0.0005/cm (base 10) when irradiated with a 193 nm laser having a fluence of at least about 0.97 mJ/cm2/pulse. In one preferred embodiment, glass articles are provided that exhibit an absorption change less than about 0.0002/cm (base 10) when irradiated with a 193 nm laser having a fluence of at least 0.97 mJ/cm2/pulse.
The fused silica articles of the present invention enable the production of lens systems exhibiting lower absorption levels within lens systems used in photolithographic equipment. The fused silica articles of the present invention also exhibit improved transmission compared to prior art fused silica articles.
Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.